“You're entirely bonkers. But I'll tell you a secret. All the best people are.”

Variant: Have I gone mad? I'm afraid so.
You're entirely Bonkers.
But I will tell you a secret,
All the best people are.
Source: Alice in Wonderland

Vorkosigan Saga, A Civil Campaign (1999)
Context: Marta blinked at him with manufactured innocence. "Kareen had it from Mark. I had it from Ivan. Mama had it from Gregor. And Da had it from Pym. If you're trying to keep it a secret, Miles, why are you going around telling everyone"?

Franny and Zooey (1961), Zooey (1957)
Context: I don't care where an actor acts. It can be in summer stock, it can be over a radio, it can be over television, it can be in a goddam Broadway theatre, complete with the most fashionable, most well-fed, most sunburned-looking audience you can imagine. But I'll tell you a terrible secret — Are you listening to me? There isn't anyone out there who isn't Seymour's Fat Lady. That includes your Professor Tupper, buddy. And all his goddam cousins by the dozens. There isn't anyone anywhere that isn't Seymour's Fat Lady. Don't you know that? Don't you know that goddam secret yet? And don't you know — listen to me, now — don't you know who that Fat Lady really is?... Ah, buddy. Ah, buddy. It's Christ Himself. Christ Himself, buddy.

Engine Summer (1979), chapter 3.2

Source: Napalm & Silly Putty

Every Last Drop, Character: Joe Pitt (narration)
Joe Pitt Casebooks

As quoted in "What a Real President Was Like: To Lyndon Johnson, the Great Society Meant Hope and Dignity" http://pqasb.pqarchiver.com/washingtonpost/doc/307079109.html?FMT=ABS&FMTS=ABS:FT&date=Nov+13%2C+1988&author=Moyers%2C+Bill+D&desc=What+a+Real+President+Was+Like%3B+To+Lyndon+Johnson%2C+the+Great+Society+Meant+Hope+and+Dignity, by Bill Moyers, The Washington Post (13 November 1988).
Attributed